The shocking revelation in front of a TV audience that included their parents took place on Telecinco's Cámbiame - Change Me - programme, a popular daytime makeover show.
After following the usual format of the show when the two, who underwent a transformation at the hands of hairdressers, make-up artists and fashion stylists, the pair announced they were very happy with their new look.
"We didn't just come here for a change in look, but also to completely change our lives. We have something to say and this is a good place to do it," said Ana, clutching the hand of her half-brother Dani.
"The big surprise is that we are not only siblings, we are also a couple," revealed Dani. "We are in love."
"We grew up separately and 20 years later found each other through social media and struck up a friendship," explained Ana in the show broadcast on April 11th. The pair share the same father but have different mothers
Watch the video of the priogramme here:
While the panel of stylists greeted the announcement with astonishment, relatives of the couple jumped up to congratulate them.
Their father hugged the pair of them and joked to his daughter "Can I call you yerna?" (The Spanish word for a daughter-in-law).
"And I'll call you suegro," she quipped back using the Spanish term for father-in-law.
Pelayo Díaz, one of the stylists on the programme said he was full of support for the couple. "Who are we to judge? Love comes above all else," he tweeted.
Incest is not against the law in Spain as long as it occurs consensually over the age of consent, which in Spain is 16 years old.
Researchers have recognised a phenomenon termed Genetic Sexual Attraction (GSA) - a powerful sexual attraction that can occur when biological relatives meet for the first time as adults.